In view of their possessing greater hardness, higher strength and lower density than metals it would seem that ceramic materials could be used in fabricating armor. Unfortunately, however, they have very low tensile strengths that make them vulnerable to any residual or tensile stresses developed during manufacture, or in defeating a projectile, that cause them to crack so as to be ineffective as armor. The present methods of ensuring that ceramics in armor will not experience tensile stresses are not reliable, are often very cumbersome and costly to implement and generally entail resorting to complicated structures not readily conformable to the shape of the vehicle or structure that needs the protection. One procedure calls for precision machined ceramic tiles and cavities in armor metal blocks. The ceramic filled cavities are then covered over with a top plate which is then generally welded to the armor metal block. Normal temperature excursions and differences in thermal expansion coefficients between the ceramic and the surrounding metal will open up a gap between these materials. Reflected tensile stresses will arise within the ceramic at this gap during ballistic impact and thereby normally lead to degraded performance of the ceramic.
Other procedures may take extensive effort to ensure that the ceramic material is in a compressive state by either wrapping the ceramic monoblocks in fiber reinforced organic composite casings before placing in the metal armor cavities, or by packing/pouring sealant compounds in the air gaps before sealing up the armor blocks. These techniques of trying to ensure that no tensile stresses are generated are also not reliable, very time consuming to implement, and ballistically inefficient. Resorting to another procedure, that of casting the metal around the ceramic often leads to shrinkage gaps at the ceramic monoblocks upon cooling the casting. Good processing control to fully eliminate these types of gaps have so far been difficult to establish. It is well known that certain metals as well as ceramics having fine grain structure can be made to exhibit what is called a superplastic property; i.e. it is pliant, by applying proper amounts of strain while within a given range of temperatures. Using a superplastic preconditioned 7475 aluminum alloy as an example, superplasticity can be made to occur by heating it to a temperature between 510.degree. C. and 520.degree. C. and slowly applying pressure at 0.05 MPa/minute that corresponds to a superplastic strain rate of 0.0001/seconds until a maximum pressure of 0.7 MPa is reached. The alloy is then cooled to room temperature while the maximum pressure is maintained. A pressure of 1000 p.s.i. equals 6.9 MPa's (megapascals). Other materials require different pressures and temperatures. In accordance with this invention ceramic tiles are located within openings formed in a layer of material, hereinafter referred to as a picture frame that exhibits superplasticity when subjected to appropriate strain while at an appropriate temperature. Preferably, layers of like material are placed in contact with opposite sides of the tiles and the picture frame so as to form an assembly in which the tiles are completely enclosed.
An aspect of this invention is the maintenance of the maximum pressure for a period of time so as to permit improved diffusion bonding of the outer layers to the tile and the picture frame. The bonding to the tile can be improved by first subjecting the tile to ion implantation or plasma enhanced coating procedures so as to provide a metallic surface.
Another aspect of this invention is to subject the assembly to further heat treatment to convert the SPF (superplastic forming) microstructure of the material of the picture frame and the layers into a higher strength microstructural form. This provides additional armor protection without any loss of mass efficiency. This post heat treatment temperature is less than that used in creating the superplastic condition to ensure that there is no diminution of the compressive stress on the embedded ceramic tile,
A less preferable armor structure of this invention that is still better than the prior art is one in which the outer layers of the assembly are eliminated in which case the pressure is applied by gas to an assembly that is enclosed within an evacuation bag.
One of the significant advantages of this invention is the fact that tile is embedded in metal that can be bent so as to provide a desired shape. The required procedures are performed to create the required compressive forces on the tile that eliminate tensile forces that may result from the impact of a penetrator. This result obtains because the metal is such that it acquires a superplastic state when subjected to changing pressure at a desired temperature. When in this state, the metal becomes intimately connected to the tile, and contractive forces occur as it is cooled down.